The invention is directed to the production of catechol and hydroquinone by hydroxylating phenol with aqueous hydrogen peroxide.
Catechol and hydroquinone are used in the manufacture of dyes, in the production of plastic as well as in the photo and plant-protective agent industry.
It is known according to German Patent 2064497 that the nuclear hydroxylation of aromatic compounds, especially of phenol, can be performed with hydrogen peroxide in the presence of a strong acid. The reaction medium should initially contain not more than 20 percent by weight water, preferably below 10 percent by weight.
The pH-H.sub.2 O of the strong acids is stated to be under -0.1, preferably under -1. Sulfuric acid and perchloric acid appear to be preferred.
However, this method is evaluated rather negatively by the owner of German Patent 2064497 itself in its later patent German 2658545. Thus, the yields of hydroxylation products with the simultaneous usage of strong acids and of complexing agents for metals like pyrophosphoric acid are supposed to be "excellent". However, the degree of conversion of the aromatic compound was under 30%. In practice, there was not more than 4 to 10% degree of conversion.
This means a limitation of the productivity of the equipment and the recycling of a significant volume of initial material.
Therefore, as high a reaction speed as possible would be important. At a given temperature and amount of water this speed would be a function of the type and amount of the acid used. Independently of the type of acid, however, it would be desirable to increase the reaction speed without increasing the amount of acid, since the latter would be lost by being washed out and, moreover, the corrosion by the strong acid also cannot be ignored.
Thus, German Patent 2658545 suggests as an improvement to the method cited above the additional usage of aromatic aldehydes such as benzaldehyde together with the catalysts and stabilizers mentioned therein. Thus, the hydroxylation of phenol and substituted phenols with hydrogen peroxide takes place in the presence of strong acids, metal complexing agents and aromatic aldehydes.
Thus, instead of two components which influence the reaction, there are now used three. These components must not only be separated from the reaction mixture and are lost, since they are non-recoverable, but the "aromatic aldehyde" component is also subject to an oxidation with hydrogen peroxide, which includes the danger of contaminating the final product.
However, higher yields of approximately 70-76%, and in two instances of over 80%, also are obtained here only if phenol/hydrogen peroxide mole ratios of 20:1 and hydrogen peroxide of approximately 85% are used. However, as in German Patent 2064497, "a significant volume of initial material must be recycled" in these cases, which requires appropriate industrial systems. The reactors themselves must also be correspondingly large.
Since the yield of catechol and hydroquinone drops considerably when the mole ratio of phenol to hydrogen peroxide is reduced in the method of German Patent 2064497--at a ratio of 10:1 it is 60% and at a ratio of 5:1 it is 47%, see example 7--the presence of a large excess of phenol appears to be a necessity when carrying out the hydroxylation with strong acids, quite aside from the fact that the strong acids such as sulfuric acid and perchloric acid cited in German Patent 2064497 only had insufficient activity as catalysts.
Thus, the assumption had already been made that the catalytic action of the strong acids could be improved by using anhydrous solutions of hydrogen peroxide, of German Patent 2410742 and German Patent 2410758; however, here too, the presence of phosphorus compounds as complexing agents was also considered to be essential. It was pointed out in addition, that the reaction occurs most rapidly at high concentrations of acid. Thus, the question of corrosion by strong acids was not solved here either.
A considerable advance compared to the methods cited was achieved by the methods of DE-PS 3308737, DE-PS3308769, DE-PS3308763 and DE-PS 3308726, in which the hydroxylation of phenol or its derivates was performed by organic solutions of hydrogen peroxide in the presence of sulfur dioxide or selenium dioxide.
In these methods, the disturbing consequences of the strong acids such as corrosion did not occur, nor was it necessary, in order to raise the activity, to use additional compounds such as aldehydes or complexing agents such as phosphorus derivates.
Although the catalysts were used in very small amounts, the reaction speed was high; very advantageous space-time yields and very good yields were obtained. Due to the very slight amount employed, sulfur dioxide and selenium dioxide also require no special separation methods.
According to the state of the art, at first, only the use of anhydrous solutions of hydrogen peroxide appeared to result in better conversions and yields. However, when using strong acids as catalyst, there was still the problem of their separation as well as the occurrence of corrosion.
The use of sulfur dioxide or selenium dioxide achieved a considerable advance here. But even these methods required anhydrous solutions of hydrogen peroxide which were supposed to contain not more than 1% by weight, preferably under 0.5% by weight water. This procedure also requires additional systems for the separation and recycle of the organic solvent.
The present invention has the task of performing the hydroxylation of phenol with sulfur dioxide or selenium dioxide in an industrially simple form but with very good yields and selectivities.